Mark E. Ander

IDB2; a Fortran program for computing extremal bounds in gravity data interpretation

It is well known that the interpretation of gravity anomaly data suffers from a fundamental nonuniqueness. No matter how complete a gravity data set may be, there are an unlimited number of subsurface density solutions compatible with it. This generally remains true even when the class of mathematically acceptable solutions is limited by imposed constraints based on physical or geologic arguments. The common practice constructing a single solution fitting, or only approximately fitting, the anomaly data is therefore of limited value.

The Qualibou caldera, St. Lucia, West Indies

Abstract Recent geological, geophysical, and hydrogeochemical studies conducted in the Qualibou area of St. Lucia, West Indies, provide new data for reevaluation of the geothermal resource and recommendation of sites for renewed drilling activities. This work supports the original hypothesis of Tomblin that the Qualibou depression is a caldera. Precaldera volcanic activity was concentrated along faults associated with regional NE- and NW-trending structures. Basaltic lavas, dated at 5.5 Ma, crop out along the western coast and are overlain by andesitic composite cones, dated at 1.2 and 0.9 Ma, which form the highest ridges of the island. Superimposed upon the andesitic cones are dacitic domes (0.25 Ma), the eroded plugs of two of these form the spectacular Pitons. The major event in this volcanic field was the intermittent eruption of the Choiseul Pumice concurrent with the formation of the Qualibou caldera (32,000 to 39,000 yrs ago). About 6 km3 (dense rock equivalent) of lithic-crystal andesitic tephra was erupted mainly as nonwelded to welded pyroclastic flows and surges. Some of these tuffs have been identified in geothermal drill holes within the 12-km2 caldera. Postcaldera eruption of dacitic tephra and dome lava (20,000 to 32,000 yrs ago) occurred from vents within the caldera and appear to be a result of magmatic resurgence. A 5.2-km-long dipole-dipole DC resistivity survey, measured along a north-south-trending line through the caldera gave apparent resistivity results similar to those obtained in previous studies. These results are compatible with a caldera substructure where low apparent resistivities ( Analysis and interpretation of hydrogeochemical data from the Qualibou caldera indicate that a geothermal reservoir underlies the Sulphur Springs area and consists of three layers: (1) an upper steam condensate zone; (2) an intermediate two-phase (vapor) zone; and (3) a lower brine zone. Measured temperatures at depth of 212°C are complemented by estimated temperatures of 250°C in the brine layer. The water chemistry of various thermal springs indicates upwelling primarily near the caldera center at Sulphur Springs, which feeds steam to steamcondensate hot springs along the northern caldera wall.

Gravity ideal bodies

The interpretation of gravity anomaly data suffers from a fundamental nonuniqueness, even when the solution set is bounded by physical or geologic constraints. Therefore, constructing a single solution that fits or approximately fits the data is of limited value. Consequently, much effort has been applied in recent years to developing inverse techniques for rigorous deduction of properties common to all possible solutions. To this end, Parker developed the theory of an ideal body, which characterizes the extremal solution with the smallest possible maximum density. Gravity ideal‐body analysis is an excellent reconaissance exploration tool because it is especially well suited for handling sparse data contaminated with noise, for finding useful, rigorous bounds on the infinite solution set, and for predicting accurately what data need to be collected in order to tighten those bounds. We present a practical three‐ dimensional gravity ideal‐body computer code, IDB, that can optimize a mesh with over 105 cells...

A regional strategy for geothermal exploration with emphasis on gravity and magnetotellurics

Abstract As part of the resource evaluation and exploration program conducted by Los Alamos Scientific Laboratory for the national Hot Dry Rock (HDR) Geothermal Program, a regional magnetotelluric (MT) survey of New Mexico and Arizona is being performed. The MT lines are being located in areas where the results of analysis of residual gravity anomaly maps of Arizona and New Mexico, integrated with other geologic and geophysical studies indicate the greatest potential for HDR resources. The residual gravity anomalies are derived by applying the concept of predicting gravity anomalies from topography. This can be accomplished by employing reductions similar to those used in some isostatic investigations, in which a regional topographic surface is used as the Bouguer reduction datum. The datum is derived by comparison of various harmonics of Bouguer anomalies and elevations of stations. Topography can be used to predict Bouguer anomalies because of isostatic compensation; the resultant anomalies can be considered high frequency residual anomalies or isostatic anomalies corrected for regional compensation. Such maps have been produced for Arizona, New Mexico, west Texas, and Chihuahua, Mexico. The main objective of the MT project is to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau and the adjacent Basin and Range Province and Rio Grande Rift. The MT survey consists of 200 sites along several long profiles with site spacing of 15–20 km. Pre-existing available MT data are being integrated with the new data. After the data are processed, a one-dimensional inversion is applied to the sounding curve and used as a starting point for 2-D modeling. Such a project and ultimate map will be of major value in studying the regional geophysics and tectonics of the southwest United States as they now apply to HDR resources in particular and geothermal resources in general. Electrical conductivity anomalies of large areal extent are of particular interest in geothermal exploration. Correlation analysis of large conductive anomalies with other geophysical, geological, and geotectonic data is being performed. Preliminary analysis of the data has suggested several major regions of anomalously shallow high electrical conductivity. Among these is the Aquarius area of northwest Arizona which is the site of a longwavelength residual anomaly low, which when modeled and correlated with other geophysical data can be shown to be possibly related to low density and high temperature in the crust at depths of 20 km or less. Preliminary analysis of MT data indicates the possible existence of a mid-crustal high electrical conductivity anomaly in this same region.

Mafic intrusion beneath the Zuni-Bandera volcanic field, New Mexico

A positive, northeast-trending gravity anomaly, 90 km long and 30 km wide, extends southwest from the Zuni uplift, New Mexico. A shallow mafic intrusion, possibly emplaced sometime before Laramide deformation, is proposed to account for the positive gravity anomaly. The Zuni-Bandera volcanic field, an alignment of 74 basaltic vents, is parallel to the eastern edge of the anomaly. Lavas display a bimodal distribution of tholeiitic and alkalic compositions and were erupted over a period from 4 m.y. B.P. to present. A residual gravity profile taken perpendicular to the major axis of the anomaly was analyzed using linear programming and ideal body theory to obtain bounds on the density contrast, depth, and minimum thickness of the gravity body. Two-dimensionality was assumed. The limiting case where the anomalous body reaches the surface gives 0.1 g/cm 3 as the greatest lower bound on the maximum density contrast. If 0.4 g/cm 3 is taken as the geologically reasonable upper limit on the maximum density contrast, the least upper bound on the depth of burial is 3.5 km, and minimum thickness is 2 km. Analysis of a magnetotelluric survey suggests that the intrusion is not due to recent basaltic magma associated with the Zuni-Bandera volcanic field. The intrusive structure has controlled the development of the volcanic field; vent orientations have changed somewhat through time, but the trend of the volcanic chain followed the edge of the intrusive structure. The intrusion has also exhibited some control on deformation of the sedimentary section.

LaCoste & Romberg gravity meter : System analysis and instrumental errors

Although the LaCoste & Romberg (L&R) gravity sensor can be used with great care to collect 1-μGal data, the sensor has a thermodynamic noise limit of 0.012 μGal—about two orders of magnitude below the present measurement precision. Hence, thermodynamic noise is not the limiting factor, and there is significant room for improvement by bringing the other perturbing influences under adequate control. We do not see any fundamental instrumental limitations to substantially improving measurement accuracy at least down to 0.1 μGal. We have improved the noise level, stability, and reliability of the L&R borehole gravity meter sensor, with the goal of reducing the total system noise to below 1 μGal. In the process of these improvements, we made several fundamental observations about noise sources within the L&R sensor, particularly related to thermal noise, electronic noise, sources of mechanical and electrical rectification errors, temporal mass variations, and instrument tares and drift. It has always been assum...

The Newtonian gravitational constant on the feasibility of an oceanic measurement

The Newtonian gravitational constant G is a fundamental parameter of physics relating the gravitational force to the product of body masses by an inverse square of the separation. G has been measured with an accuracy of about 7 parts in 105 in individual laboratory experiments [e.g., Luther and Towler, 1982], but the consistency of all modern laboratory measurements is only about 7 parts in 104 (Figure 1), making it one of the most poorly determined physical constants of nature [cf. Cohen and Taylor, 1986; Gillies, 1987]. For the past century, virtually all experiments to measure G have been conducted on a scale (i.e., separation between test masses) of 50 cm or less, using a modification of the Cavendish balance. In recent years there has been increasing interest in determinations of G over larger scales than can be achieved in the laboratory. Some theoretical attempts to combine gravity with the other forces of nature predict the existence of a fifth force in addition to the classical gravitational, electromagnetic, weak, and strong forces. The fifth force would produce departures from Newtonian or inverse square law gravity at mass separations of tens of meters to tens of kilometers. Geophysical experiments are uniquely suited to measure the gravitational constant at these scales, and in this paper we outline the advantages of conducting such an experiment in the ocean.

Regional gravity investigation of Honduras, Central America

Abstract A regional gravity study of Honduras was performed as part of a major study of the geothermal resource potential of Honduras. This study was conducted by Los Alamos National Laboratory, in cooperation with the Honduras government. Regional offshore free-air and onshore Bouguer gravity maps, and residual/isostatic gravity maps of Honduras and surrounding regions were produced. From these data several regional crustal and upper mantle models were produced. These models pass through two local geothermal sites, Platanares and San Ignacio. The regional geologic and tectonic implications of the models and their relevance to the geothermal potential of Honduras and to six well known geothermal sites in particular are examined. No obvious regional structures observed in the gravity data can explain the thermal enhancement in general or the specific geothermal sites. More local tectonic or structural conditions must control the distribution of the thermally enhanced areas.

Analysis of the Greenland icesheet experiment in terms of new gravitational forces

Abstract We present an analysis of the anomalous gravitational attraction found in the Greenland icesheet experiment. If the anomalous gradient is due to the existence of a single new, attractive, force, then the force has a range between 225 m and 5.40 km and a coupling constant between 2.4% and 3.5% that of newtonian gravity, with a best fit of about 805 m and 3.1%, respectively. From our procedure, the chi-squared analysis describing the quality of any fit to parameter values can be immediately scaled if a fraction of the gravitational anomaly is actually due to density anomalies in the bedrock.

Geophysical investigations of the Platanares geothermal site, Honduras, Central America

Abstract Geophysical studies were carried out in Platanares in Western Honduras, as part of an investigation of the geothermal resource potential of Honduras by Los Alamos National Laboratory in cooperation with the Honduran government and the U.S. Geological Survey. Geological and geochemical studies later followed by exploratory bore-holes indicate this area of rugged topography has the best geothermal potential in Honduras. In order to better understand the geology of the area, gravity, audiomagnetotelluric and telluric surveys were carried out. The site is in a graben of Tertiary volcanics and Cretaceous red beds, with numerous hot springs along faults in the center of the graben. Gravity mapped the thickness of low-density tuffs and the position of associated normal faults. It also indicated the presence of persuasive, shallow, highly silicified, high-density tuffs and gravels. The electrical surveys mapped shallow, high-resistivity silicified zones and faults as low-resistivity zones. The red beds below the tuffs are considered to contain the geothermal reservoir, but their maximum thickness could not be determined by these surveys due to the lack of density contrast and the high surface electrical conductivities. Three geologic cross-sections constructed by others have been re-evaluated using gravity modeling and analysis of eletrical anomalies, showing general agreement but in detail significant differences.